The idea that the efficacy of human physical and intellectual performance is a function of brain size has been debated without resolution for more than a century. Despite the considerable scientific (and political) rancor generated by past exploration of this problem, the issue remains a central one in both basic neurobiology and human development. As explained in the application, attempts to link a complex and controversial measure of performance such as IQ to overall brain size have for the most part been ill advised. Nonetheless, the question of whether better performance in any domain of human behavior depends on a greater amount of anatomically defined brain space devoted to that function remains an important and unsolved problem in modem neuroscience. Here we propose to examine this question by determining the relation between human visual ability and the size of the primary visual cortex. The visual performance of healthy young adults will be scored by an evolving battery of behavioral tests that assess a variety of major visual functions. The extent of the primary visual cortex (VI) will then be measured in the same subjects by non-invasive brain imaging (fMR1 in conjunction with cortical flattening methodology). In this way, we will determine whether individual human performance in a well-defined domain is related to a measurable area of cerebral cortex that is fundamental to carrying out those functions. Successful completion of this work speaks to a long-standing controversy pertinent to both clinicians and neurobiologists interested in how the function of the human brain is related to its structure. The results should also establish a paradigm for evaluating other anatomically defined brain areas as non-invasive brain imaging technology continues to improve to the point where they can-like V I-be accurately measured.